fix principalSturmHabichtSequence

hspray.cabal

@@ -1,7 +1,7 @@
 name:                hspray
 version:             0.5.2.0
 synopsis:            Multivariate polynomials and fractions of multivariate polynomials.
-description:         Manipulation of multivariate polynomials over a commutative ring and fractions of multivariate polynomials over a commutative field, Gröbner bases, resultant and subresultants, and greatest common divisor. It is possible to deal with multivariate polynomials whose coefficients are fractions of multivariate polynomials, and they can be interpreted as parametric polynomials with symbolic parameters.
+description:         Manipulation of multivariate polynomials over a commutative ring and fractions of multivariate polynomials over a commutative field, Gröbner bases, resultant, subresultants, Sturm-Habicht sequence, and greatest common divisor. It is possible to deal with multivariate polynomials whose coefficients are fractions of multivariate polynomials, and they can be interpreted as parametric polynomials with symbolic parameters.
 homepage:            https://github.com/stla/hspray#readme
 license:             GPL-3
 license-file:        LICENSE

src/Math/Algebra/Hspray.hs

@@ -5,7 +5,7 @@ Copyright   : (c) Stéphane Laurent, 2022-2024
 License     : GPL-3
 Maintainer  : [email protected]
 
-Deals with multivariate polynomials on a commutative ring. 
+Deals with multivariate polynomials over a commutative ring. 
 See README for examples.
 -}
 {-# LANGUAGE BangPatterns #-}
@@ -192,7 +192,7 @@ module Math.Algebra.Hspray
   , substituteSpray
   , composeSpray
   , evalSpraySpray
-  -- * Division of a spray
+  -- * Division of sprays
   , sprayDivision
   , sprayDivisionRemainder
   , pseudoDivision
@@ -2636,7 +2636,7 @@ principalSturmHabichtSequence var spray
     sHS = sturmHabichtSequence var spray
     permutation  = [d - var + 1 .. d] ++ [1 .. d - var]
     permutation' = [var + 1 .. d] ++ [1 .. var] 
-    jcoeff j = if isZeroSpray sHSj 
+    jcoeff j = if isZeroSpray sHSj || j >= length coeffs
       then zeroSpray
       else permuteVariables permutation' (coeffs !! j)
       where

tests/Main.hs

@@ -838,6 +838,30 @@ main = defaultMain $ testGroup
           ]
         )
 
+    , testCase "principalSturmHabichtSequence" $ do
+      let
+        x = qlone 1
+        y = qlone 2
+        p = x^**^4 ^*^ y ^+^ y^**^2
+        psh1 = principalSturmHabichtSequence 1 p
+        psh2 = principalSturmHabichtSequence 2 p
+      assertEqual "" 
+        (psh1, psh2)
+        (
+          [
+              256 *^ y^**^10
+            , zeroSpray
+            , zeroSpray
+            , 4 *^ y
+            , y
+          ],
+          [
+              x^**^8
+            , constantSpray 2
+            , constantSpray 1
+          ]
+        )
+
     , testCase "sturmHabichtSequence & principalSturmHabichtSequence" $ do
       let
         x = qlone 1